In mobile communications system, when a mobile terminal, such as a UE in a Long-term evolution (LTE) system, leaves a source cell and enters another cell, the signals it receives from the source cell become weak increasingly, and the signals from the inbound cell the mobile terminal is entering will become more and more strong. In order to ensure the communication quality of UE and realize the mobility management in a connected status, a handover from the source eNB to an eNB with stronger signals can be performed for the mobile terminal; that is, the mobile terminal is switched from a source cell to a target cell. Usually, an eNB for providing resources for a mobile terminal before handover is called a source eNB, and an eNB for providing sources for a mobile terminal after handover is called a target eNB.
Various types of mobile communications system can use their handover rules to realize handover for mobile terminals, respectively. Taking the Long-term evolution (LTE) system as an example, the handover of the mobile terminal includes: the source eNB selects a target eNB from all neighboring eNBs in accordance with Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ) or other received signal parameters etc., sends a handover request to the selected target eNB, and informs the mobile terminal for handover upon receipt of a handover request acknowledge signal fed back by the target eNB in the target cell. Then, the mobile terminal establishes a connection with the target eNB through a random access mode. However, if the mobile terminal does not have service authority in the target eNB or the load of the target eNB is too heavy, the target eNB will not feed back a handover request acknowledge signal to the source eNB, and the handover is failed. In addition, if a plurality of mobile terminals initiate random access to the target eNB at the same time, that is a random access collision occurs, or system information of the target eNB stored in the mobile terminal becomes invalid, the random access initiated by the mobile terminal to the target eNB fails, which would lead to a handover failure. In order to recover the handover as soon as possible for enabling the mobile terminal to continue enjoying the service, a handover processing approach based on prepared eNBs is put forward in the LTE. In this approach, after handover fails, the mobile terminal selects a suitable cell, and establishes a Radio Resource Control (RRC) connection with the eNB of the suitable cell. If the eNB of the suitable cell is one of a source eNB, a target eNB or a prepared eNB, the RRC connection can be successfully established, and the handover failure recovery is in turn successful. If the eNB of the suitable cell is none of the above mentioned three types of eNBs, it means that the context relation information of the mobile terminal does not exist in the eNB of the suitable cell, and then the RRC connection establishment is failed and the handover failure can not be recovered.
FIG. 1 is a flowchart illustrating a handover processing method in an existing LTE. As shown in FIG. 1, the method includes:
In step 101, a source eNB issues measurement configuration information to a mobile terminal, and the mobile terminal reports to the source eNB a measured RSRP of neighboring cells.
In this step, the measurement configuration information issued by the source eNB to the mobile terminal includes at least measurement objects, measurement report configuration, wherein the measurement objects are neighboring cells of the cell the source eNB corresponds to, which can be of various types and not be limited to such as working carrier frequency and wireless access technology. The measurement report configuration refers to a condition that the mobile terminal reports a measurement report. Such as if the measured RSRP of an neighboring cell is higher than a predefined threshold, the mobile terminal will reports the measured RSRP of the neighboring cell to the source eNB.
The mobile terminal measures the measurement objects in accordance with the received measurement configuration information, and reports to the source eNB RSRPs corresponding to the cells who meet the measurement report configuration, i.e., the measurement report cells. Here, the mobile terminal can also measure parameters such as RSRQ of a measurement object, and report the measured RSRQ.
In step 102, the source eNB makes a handover decision, and performs step 103 when it is determined that a handover trigger condition is currently met.
For example, when the reported cell is an intra frequency cell, if one of the following scenarios appears, that is the RSRP of the source cell is lower than a predefined threshold 1 and the RSRP of an neighboring cell is higher than a predefined threshold 2, and the RSRP of the neighboring cell is offset higher than that of the source cell, the source eNB can determine that a handover trigger condition is met in the current scenario, and then continue to perform a handover in subsequent steps. If the source eNB determines that a handover condition is not satisfied in the current scenario, the procedure is ended. When the reported cell is an inter frequency cell, the source eNB further needs to consider inter frequency hysteresis in the handover decision process.
In steps 103-104, the source eNB sends a handover request to each of the neighboring eNBs of the reported cell, any of the neighboring eNBs receiving the handover request feeds back a handover request acknowledge signal when determining that the neighboring eNB can be accessed by the mobile terminal, and carries out a handover preparation.
When sending a handover request to each of the neighboring eNBs, the source eNB may carry context relation information of a mobile terminal into the handover request, such as user service class and user ID corresponding to the mobile terminal.
After receiving the handover request from the source eNB, a neighboring eNB can make an access decision according to admission control criteria such as service authority and load. If it is determined that the handover request of the mobile terminal is acceptable, a handover preparation is carried out via the following ways such as allocating an idle random access preamble and resource data resources for the mobile terminal.
In step 105, the source eNB will store in itself information of neighboring eNBs corresponding to the handover request acknowledge signals as information of prepared eNBs, select a target eNB from them, and send a handover command including target eNB information to the mobile terminal.
The information of prepared eNBs stored in the source eNB may be a cell ID corresponding to the eNB. Thereafter, the source eNB selects a target eNB from the prepared eNBs based on a principle of the largest RSRP or a principle of user preferences, for example, selecting prepared eNB 1 as a target eNB.
In step 106, a random access initiated by the mobile terminal to the target eNB is failed.
In step 107, the mobile terminal selects a suitable cell.
In this step, the mobile terminal performs cell selection by use of such as measurement. As a result of a failure of the mobile terminal to access the target eNB in the foregoing step 106, the mobile terminal will not update its own configuration information into configuration information corresponding to the target eNB, but retain the configuration information previously set by the source eNB for the mobile terminal.
In step 108, the mobile terminal establishes an RRC connection with an eNB of a selected suitable cell, and the handover process is ended after the RRC connection is successful established.
When the eNB corresponding to the suitable cell selected by the mobile terminal is a prepared eNB, it is more likely that the eNB has already stored context relation information of the mobile terminal, and there is a higher probability of random access success. Therefore, an ideal situation is the prepared eNB is selected as a suitable cell eNB, for example, prepared eNB 2 is used as a target eNB at this time.
Then, an existing handover processing procedure based on prepared eNBs is completed.
During the above-mentioned handover processing procedure based on prepared eNBs, the source eNB issues a handover request to a number of neighboring eNBs at the same time in the early stage. When determining via an admission control that the handover request of the mobile terminal is acceptable, a neighboring eNB having received the handover request feeds back a handover request acknowledge signal to the source eNB. Also, a neighboring eNB will reserve part of its resources for the mobile terminal, and the reserved resources will be in real use when the prepared eNB is selected by the mobile terminal as a target eNB. If a neighboring eNB is waiting for handover of multiple mobile terminals at the same time, that is resources are reserved for each of the mobile terminals, the available resources of the neighboring eNB may be reduced. Then, when any new handover request arises, the neighboring eNB will not be able to provide adequate resources which may result in an admission control failure, and also result in lower successful handover probability of the mobile terminal. On the other hand, the prepared eNB can not learn whether the mobile terminal has successfully switched to the target eNB, and the target eNB also does not inform the prepared eNB to release the reserved resources. If the prepared eNB unilaterally releases the reserved resources corresponding to the mobile terminal, the mobile terminal will not be able to handover to the prepared eNB any longer, which may also affect the successful handover probability of the mobile terminal.